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Exposure is the control of light reaching the image sensor via the combination of aperture and shutter speed. The sensor's sensitivity to light can also be adjusted via the ISO setting for additional control over how bright the recorded image is.
To determine what exposure will give the desired image brightness, the camera will meter the light radiating from the subject and calculate the exposure settings required for the image sensor to effectively record the scene.
Irrespective of how much ambient light is available, you can safely assume that unless in total darkness there will be enough light out there to capture and therefore record an image.
The final quantity of light that reaches the image sensor is determined by several primary factors (and some secondary ones too) these are as follows:
Secondary factors are,
The Aperture is a variable size hole that controls the amount of light that can pass through the lens when you press the shutter. The larger the area of this hole, the more light will reach the sensor whilst the shutter is open.
Focal Ratio (F-Number or F.Stop)
This gives use the F-Numbers or F Stops - The numbers with the sequence F2, F4, F5.6, F8, F11, F16, F22 etc etc that you find on you lens.
IMPORTANT : Being ratios, F Number values get bigger as the aperture size gets smaller! So F2 is a much larger aperture than F8 (in the same way as 1/2 a pizza is a much bigger slice than 1/8th of a pizza!)
The focal ratio ensures that the amount of light (and therefore the exposure) is constant for a given F Number. So for example, if you have good exposure at a focal length of say 50mm and aperture of F5.6 if you then "zoom in" to a focal length of 200mm - so long as you maintain the aperture at F5.6 you can be sure the exposure will still be good.
If you would like a more thorough explanation of how aperture, focal length and F-numbers affect digital photography exposure please read Shedding the light on f-numbers.
This impacts upon the quantity of light reaching the sensor by determining the period of time (normally measured in seconds) that light is permitted to pass through the aperture. A fast shutter speed (ie a short length of time) therefore allows less light than a slow shutter speed (ie a long period of time). Depending on the type of camera, shutter speeds can range anywhere from 1/8000 of a second to 30 seconds. Some cameras also have what is called a BULB setting which allows the photographer to manually determine how long the shutter is open.
Just as aperture affects depth of field as well as exposure, the choice of shutter speed is also important for achieving the desired image quality and creative affect.
Unlike aperture and shutter speed, the ISO setting doesn't actually control the amount of light reaching the image sensor. Rather, it electronically changes the way the image sensor manages the light it receives. In simplified terms, each time the ISO setting is doubled the image sensor's sensitivity is increased such that it will record the image as though the actual light itself had been twice as high.
Unfortunately, raising the ISO has a detrimental effect upon image quality (because the higher the sensitivity of the sensor the greater the susceptibility for each pixel to record digital noise . The range of ISO settings over which image quality remains acceptable is improving rapidly as digital technology advances.
The ability to have the effect of increasing exposure without the need to actually increase the amount of light via larger aperture and/or slower shutter speeds has numerous benefits. For example in situations where traditionally the only way to get enough light reaching the sensor was either via a very large aperture lens (ie bulky, expensive and shallow depth of field) or a slow shutter speed (blurred images due to camera shake or subject motion). These days with a good DSLR camera particularly when combined with an image stabilization system it is possible to capture good quality images in these conditions, even without the need to use a tripod or flash.
That said, it's still best practice to keep "noise" to a minimum by whenever possible adjusting the exposure primarily via aperture and shutter speed and only increasing ISO if the exposure (or creative effect) cannot be achieved via those settings alone.
The highest ISO setting that you ultimately use is dependent on
As you know, your final image is the result of how much light is converted into data at each and every pixel (if you weren't aware of this, no problem, click here for BASICS OF DIGITAL PHOTOGRAPHY). So, in order for the final image to be perfectly exposed, you would ideally determine the exact amount of light needed at each pixel and adjust the camera settings accordingly. Now, if you have a fairly typical digital camera of maybe 10megapixel or above - this would mean simultaneously setting 10million individual aperture, shutter speed and ISO settings for each and every photo! Clearly this is a practical impossibility. Fortunately, in reality it is not necessary to do this, as dividing the image into a few zones (typically around 35) and then averaging the amount of light needed by each is typically good enough to calculate the exposure needed. (Don't worry, your camera's on board computer does this calculation for you).
Sometimes however, it might be appropriate to consider the optimum light needed to expose some parts of the image more than others (particularly if your primary subject occupies only a small part of the frame and is significantly more dimly (or brightly) lit than the rest of the image. In these cases you will want to use partial metering. In partial metering rather than the average light needed for ALL zones of the image being computed, greater weight is given to the light demanded at the important part of the image and the exposure is calculated with this in mind. This will of course result in the other areas of the image being over or underexposed - nevertheless this is preferable to the main subject being too light or dark.
Most modern cameras offer several metering options which average the exposure needs of the image in a variety of ways. These are EVALUATIVE, PARTIAL, SPOT, and CENTRE WEIGHTED. Consult your camera instructions for details of how each is used to determine exposure and for an indication of when to use each.
There is no such thing as the CORRECT digital photography exposure.
The more appropriate question that YOU the photographer should ask yourself is, "will or did (if you have already taken the photo and are reviewing it on your camera screen) the exposure controls selected, yield the image result desired in respect of brightness of the subject, depth of field, sharpness and image quality?
When the answer to all these questions is YES, then the exposure is "correct" for that particular image.
Auto versus programmeable and manual exposure.
Your camera will probably have various exposure mode options as follows,
So which should you use? Well thats where YOUR JUDGMENT (and skill, experience and knowledge) comes into play. Left on full auto, your camera will be programmed to give a result reasonably expected to be desirable for general everyday photos. ( ie with mid range aperture and shutter speed and ISO levels such that the final image is relatively balanced for brightness over the entire frame.
The scene modes will take into consideration other factors normally relevant for that type of scene. For example, SPORTS mode takes into account that fast moving subjects need a fast shutter speed to avoid motion blur - and so will obtain the exposure with faster shutter speeds (and the correspondingly larger apertures), whereas LANDSCAPE mode, will consider the desire for a large depth of field to ensure both near and far objects are in focus - and will therefore use small apertures and therefore slower shutter speeds.
The more familiar you are with the concepts of photography and the more familiar you are with the range of settings of YOUR camera (and how readily you can access and set them), the more often you will find yourself taking control of the camera's programmable options.
This will come with practice - and as said elsewhere on this site, the real beauty of digital photography is that after the initial investment in equipment, taking pictures is essentially for free and of course its possible to instantly review your pictures to see if your settings are giving the results you want.
However, if you are unsure and are shooting an event in real time that is important, unlikely to wait for you, or to be repeated - then there is no shame in resorting to those scene modes or even full auto - after all they are designed based on the input of photography professionals with years of experience to give the best result in the majority of situations.
Don't worry with time and practice you WILL master digital photography exposure!
Exposure is the control of light reaching the image sensor via the combination of aperture and shutter speed. The sensor's sensitivity to light can also be adjusted via the ISO setting for additional control over how bright the image is recorded.
To determine what exposure will give the desired image brightness, the camera will meter the light emanating from the subject in and calculate the exposure settings required to allow the image sensor to effectively record the scene.
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